Fitting arrangements for effecting a compressive sealed engagement to the ends of non-threaded plastic tubing are well known. Such fitting arrangements are disclosed in U.S. Pat. No. 4,799,717 and can be understood by referring to FIG. 1 herein. The known fitting arrangement 10 conventionally employs a fitting body 12 which has a tubular collar 14 which telescopes onto an end portion 16 of a plastic tube 18. The collar 14 is threadably engaged with an annular nut 20 which surrounds the collar. The annular nut 20 and collar 14 have an opposed interior flared seat 22 and tapered bore 24, respectively. A ferrule ring 26 and a gripper ring 28 are deposed in surrounding relationship to the plastic tube 18 between the opposed flared seat 22 and tapered bore 24 surfaces, respectively.
The ferrule and gripper rings each have exterior surfaces which cooperate with the adjacent surface of the flared seat and tapered bore. The gripper ring 28 also has interior teeth 30 adapted to bite into the plastic tube 18. Threading the nut 20 onto the collar 14 causes axial compression of the ferrule and gripper rings 26 and 28 between the opposed-flared seat 22 and tapered bore 24, and effects radial compression of these rings into sealing engagement with the plastic tube. Fitting arrangements of this type are extensively used in conjunction with plastic tubes which confine and transport dangerous fluids, for example, fluids that are at a high temperature or which are corrosive, such as strong acids or bases. Such dangerous fluids are used extensively in the semiconductor processing industry.
Fitting arrangements of the above-mentioned type are not without their disadvantages, however, as the plastic tubing is only compressively engaged within the collar by the action of the ferrule and gripper ring against the relatively deformable tubing sidewall. Accordingly, over time and exposure to temperature cycling, the sidewall of the plastic tube may deform radially inwardly in response to the inwardly directed compression force of the ferrule and gripper rings. This sidewall deformation may cause the plastic tube to loosen from its engagement within the collar. Depending on the extent of sidewall deformation, a sufficient gap may be created between the collar 14 and annular nut 20 that can not be accommodated by the interaction of the ferrule and gripper rings, thereby forming a leak path for the transported fluid to exit the attachment fitting 10 from the tube end 16, along the collar 22, past the tapered bore 24, and into the surrounding environment. Additionally, any fluid that is leaks past the collar 22 may pass from the fitting arrangement to the surrounding environment via the threaded coupling between the fitting body 12 and the annular nut 20.
Fitting arrangements of the above-mentioned type include a space 32 that exists between the tapered bore 24 and the outside wall of the tube 18. Upon deformation of the tube sidewall, due to the radially directed compression force of the ferrule and gripper rings against the tube, the transported fluid is allowed to pass from the end portion 16 of the tube along the collar 14 and can collect within the space 32 to form a hold-up volume of the fluid within the fitting arrangement. The formation of such a hold-up volume within the fitting arrangement is not desirable as the collected fluid may cause deterioration of surrounding fitting arrangement members. Additionally, such collected fluid may be a source of contamination if the transported fluid being routed by the tubing is changed to a different type, or may be the source of bacterial contamination in pure water systems.
Fitting arrangements such as those described above for use with a single plastic tube can also be used with concentric inner and outer plastic tubes as is disclosed in U.S. Pat. No. 4,871,196, and as described with reference to FIG. 2. The fitting arrangement 34 includes an inner plastic tube 36 having an end portion 38 engaged within a collar 40 of an annular fitting body 42. The collar 40 is threadably engaged with an intermediate annular body 44 that surrounds the collar 40 and the inner plastic tube 36 therein. A first ferrule 46 and first gripper ring 48 are positioned circumferentially around an outside surface of the inner tube between adjacent first flared seat 50 and first tapered bore 52 surfaces, respectively. Threaded engagement of the intermediate annular body 44 and the annular fitting body 42 causes the first flared seat 50 and first tapered bore 52 to be brought axially together, thereby effecting inwardly directed compression of the first ferrule 46 and first gripper ring 48 against the sidewall of the inner tube 36 to form a first attachment fitting.
The fitting arrangement 34 includes an outer plastic tube 54 having an end portion 56 engaged within a collar 58 of the intermediate annular body 44. The collar 58 is threadably engaged with an annular body 60 that surrounds the collar 58 and the outer plastic tube 54. A second ferrule 62 and second gripper ring 64 are positioned circumferentially around an outside surface of the outer tube 54 between adjacent second flared seat 66 and second tapered bore 68 surfaces, respectively. Threaded engagement of the annular body 60 and the intermediate annular body 44 causes the second flared seat 66 and second tapered bore 68 to be brought axially together, thereby effecting inwardly directed compression of the second ferrule 62 and second gripper ring 64 against the sidewall of the outer tube 56 to form a second attachment fitting. The second attachment fitting is positioned concentrically around the inner plastic tube 36.
The use of fitting arrangements which accommodate concentric inner and outer plastic tubes in sealed engagement is desirable because the outer tube provides containment for any leakage of fluids from the inner tube through which the fluids are being transported. Additionally, if desired, heated or cooled fluids can be transported through the outer tube to heat or cool fluids flowing through the inner tube.
However, the use of the concentric fitting arrangements described above that incorporate the gripper and ferrule ring type fittings possess the same inherent problems that were described with regard to the single tube fitting arrangement; namely, leakage due to the deformation of the tubing sidewalls and unwanted collection of transport fluids in spaces 70 and 72 that exist between the annular fitting body 42 and the intermediate annular body 44, and the intermediate annular body 44 and the annular body 60, respectively. Although leakage of fluid transported within the inner tube can arguably be contained by operation of the concentric outer tube, transport fluid contained within the outer tube can leak from the second attachment fitting to the surrounding environment in the same manner as described above for the single tube fitting arrangement.
Single tube fitting arrangements that do not make use of ferrule and gripper rings but, rather, incorporate compression-type attachment members are also known in the art. Such fittings are described as flared-type fittings and are used to seal the end of a single plastic tube. In the flare-type fitting arrangement, a sidewall portion of a flared portion of a plastic tube is pinched between a first attachment member positioned at a location along an inside diameter of the tube and a second attachment member positioned adjacent the first member at a location along an outside diameter of the tube. The first and second attachment members are threadably engaged with one another and tightened to cause the members to be brought axially together, thereby sandwiching the wall of the flared portion of the tube therebetween and forming an attachment fitting. The advantage of using the flared-type fitting is that the attachment fitting does not depend on the compressive engagement against a deformable member, as is the case with the above-described ferrule/gripper-type assemblies. Rather, the attachment fitting is formed by sandwiching the tube sidewall between two non-deformable attachment members.
Although the flared-type fitting reduces the problem of leakage about the attachment fitting, in view of the aggressive nature of the caustic chemicals and process conditions that are used in the semiconductor manufacturing industry, it is desirable that a secondary containment be used in conjunction with the tubing and attachment fittings for purposes of minimizing any health danger or environmental hazard that may result upon leakage of such chemicals from the tubing and/or the attachment fittings to the environment.
It is, therefore, desirable that a fitting arrangement be constructed to provide a sealed containment system for an attachment fitting and related tubing that is used to transport process chemicals. It is desirable that the fitting arrangement provide sealing engagement of both the fluid transport tube and a secondary containment member in a manner that is not prone to leakage due to its depending on a ferrule/gripper ring engagement against a deformable tube wall. It is also desirable that the fitting arrangement not promote collection and/or entrapment of transport fluid during use, and that the fitting arrangement can be manufactured in a manner that is economically feasible and from materials that are widely available.